Page 134 - Engineering Plastics Handbook

108 Engineering Plastics

time contributes to an enlargement of particle size. However, too long a
residence time may induce a discoloration of ABS resin.
The coagulated ABS slurry is centrifuged to remove water and then
washed to remove the impurities. The removed water is purified through
the filtering apparatus and then recycled. The water content after drying
is kept below 2%. The fluidized-bed type of dryer is generally used.

Mass polymerization process
According to the mass process, polybutadiene rubber is dissolved in the
mixed solution of styrene and acrylonitrile monomers, and then the reac-
tion proceeds to prepare the ABS resin. For polybutadiene rubber, the
method for grinding and then adding the rubber is utilized. Contrary to
the emulsion process, the particles of polybutadiene rubber are not pres-
ent before the reaction, and therefore, the shape and distribution of par-
ticles appear to be different from those of the emulsion process. The
molecular weight of polybutadiene rubber to be used is about 200,000, with
the cis-1,4 ratio being about 40%. The amount of polybutadiene rubber used
should be kept to 20% due to the problem related to a viscosity of poly-
merization solution.
The radical prepolymerization occurs in the oil-in-oil emulsion state. As
the reaction proceeds, the copolymer of styrene and acrylonitrile is pro-
duced, and when the ratio between the produced copolymer and polybu-
tadiene rubber reaches 1:1, the phase inversion occurs. Thereafter,
styrene/acrylonitrile copolymer is present in the continuous phase, and
polybutadiene rubber is present in the dispersed phase. In general, the
phase inversion is terminated when the polymerization conversion rate
reaches about 15%. The particle size is varied depending on the change in
stirring rate, the viscosity ratio between the rubber phase and the
monomer phase, and the interfacial tension.
The grafting rate is affected by the initiator types, the kinds of molecu-
lar weight regulators, and a difference in solubility of two monomers in the
rubber phase and the matrix phase. Due to such difference in solubility,
the grafting rate is lowered and any difference in the composition between
rubber and the matrix is created.
After the prepolymerization step, as the reaction proceeds, the molecu-
lar weight and molecular weight distribution are controlled by the tem-
perature, the kinds and added amounts of the initiator, the content of the
solvent, the added amounts of the molecular weight regulator, etc. To pre-
vent a cracking of the particles, it may be necessary to increase the cross-
linking density of rubber. In such a case, the reaction temperature and time
can be controlled to obtain the desired properties.
The molecular weight of the final product is usually around 100,000, and
the size of rubber particles is about 1.0 to 2.5 µm. Various particle size dis-
tributions of rubber can be obtained by using two kinds of prepolymers.